The investigation into Russian intelligence activities by the Senate Select Committee on Intelligence turned two years old, without fanfare, last month.

For almost as long, the inquiry, led by Republican Chairman Richard Burr of North Carolina and Vice Chairman Mark Warner, a Virginia Democrat, has been held up as the last bastion of bipartisanship in Washington.

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After a parallel investigation divided the House Intelligence Committee last year, the Senate's probe has been under intense pressure to offer a single set of findings.

Burr, who is known in Senate hallways for his preference to go sockless and the two-fingered hook that often bears his jacket, has spoken little about the probe he leads. But he thinks deeply about how its conclusions should be presented. And he acknowledges now that the investigation is broader, and perhaps more consequential, than it has long been thought to be.

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Committee Chairman Sen. Richard Burr arrives for a meeting of the Senate Select Committee on Intelligence on Aug. 16, 2018, in Washington, D.C.

"I'm not going to tell you that what we set out to do — which was to understand what happened in '16 — is what's extended the life of the investigation," Burr said in a rare interview with CBS News. "I think it's a better understanding of what happened and how coordinated and organized the effort was."

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He spoke in his second-floor chambers in the Russell Senate Office Building, where multiple deer mounts circle the ceiling and a non-working fireplace is stacked with real wood. Burr, 63, had angled himself in a worn leather chair, legs extended, and tented his fingers around the edges of a coffee cup. He spoke quietly, with occasional long pauses as he considered his responses.

"We'll be judged at the end of this on the product that we produce," he said solemnly. "We'll also be...